1. Field of the Invention
The present invention relates to an image sensor being a photographing apparatus, and more specifically to a semiconductor image sensor.
2. Description of the Related Art
There are conventionally known as photographing devices CCD (Charge Coupled Device) image sensors and MOS (Metal Oxide Semiconductor) image sensors. The CCD image sensors are prevailing at present. Compared with the CCD image sensors, however, the MOS image sensors have the following advantages: the MOS image sensors require less power consumption, and includes an optical detection section and its peripheral circuit which can be integrated on the same chip.
As illustrated in FIG. 6, the aforesaid MOS image sensor includes a vertical scanning section 101, a horizontal scanning section 102, and a cell array 103. The vertical scanning section 101 outputs DC voltage V1 and a readout signal S2 to a cell array 103, and the horizontal scanning section 102 outputs a readout signal S2 to the cell array 103.
The cell array 103 converts incident light to an electric signal, and outputs a signal representing a picture image (hereinafter, referred to as a picture image signal ),and includes m.times.n conversion cells 111.sub.ij (i=1 to m, j=1 to n), and regulated power supplies 112.sub.1 to 112.sub.m, as illustrated in FIG. 6.
The regulated current supplies 112.sub.1 to 112.sub.m, supply a regulated current to signal lines 113.sub.1 to 113.sub.m, respectively. The conversion cells 111.sub.ij arranged in a matrix include transistors each of which is an enhancement N channel MOSFET (Field Effect Transistor) and a photodiode 124 for converting incident light to an electric signal, as illustrated in FIG. 7.
The vertical scanning section 101 vertically scans the cell array 103. The vertical scanning section 101 outputs DC voltage V1 and a high level reset signal S1 to the conversion cells 111.sub.ij to 111.sub.mj that are an object of vertical scanning. Hereby, a photodiode 124 of the conversion cell 111.sub.ij is initialized. More specifically, the transistor 121 is switched on with the high level reset signal S1, and the DC voltage V1 is applied to the photodiode 124.
Consequently, the photodiode 124 stores electric charges thereon, and a voltage at a connection point (hereinafter, referred to a node M) between the photodiode 124 and the transistor 122 becomes the DC voltage V1. The photodiode 124 is therefore initialized.
After completion of initialization of the photodiode 124, when a light is incident on the photodiode 124, electric charges stored in the photodiode 124 are reduced in response to the intensity of the incident light to a lower voltage at the node M. The voltage at the node M is a conversion signal that is a result of the conversion of the incident light.
Thereafter, the horizontal scanning section 102 horizontally scans the cell array 103. At this time, a high level readout signal S2 is inputted in order into the conversion cells 111.sub.ij to 111.sub.jm that are the object of the horizontal scanning. Hereby, the conversion signal amplified by the transistor 122 is outputted to the signal line 113.sub.i after passage through the source follower transistor 123.
When the vertical/horizontal scanning for the conversion cells 111.sub.11 to 111.sub.m1 and the conversion cells 111.sub.1n to 111.sub.mn is finished, the horizontal scanning section 102 outputs a picture image signal.
The prior art technique suffers from the following difficulty. Each conversion cell 111.sub.ij of the cell array 103 outputs the amplified conversion signal to the signal line 113.sub.i. The amplification is achieved with the transistor 122 of the conversion cell 111.sub.ij.
In contrast, the transistor 122 has a threshold, and further has an amplifying function for input voltage above the threshold. The threshold is however varied depending upon fine differences among dozes of doped impurities, and slight differences of gate configurations produced upon manufacturing. In other words, the transistors 122 have different thresholds which provide variations of the threshold of the transistor 122. Thus, noise of a fixed pattern due to the transistor 122 is generated on the aforesaid picture image signal.